The present invention relates to a waveguide structure, and more particularly, it relates to a waveguide structure capable of highly unifying a branching ratio and a coupling ratio.
Broadly prevailing PCs and Internet services for recent years have spurred the market to be active due to rapidly increased needs for transmission of bulk data, and in order to meet the needs, there are further needs for optical waveguides of higher beam propagating speed and reduced optical transmission loss. The optical waveguide is used as an optical interconnection for optical transmission, and an optical device such as a beam splitter (i.e., coupler) is essential as one of the basic components of the optical waveguide. The beam splitter serves as a combining waveguide (coupler) when reversed in the direction of light transmitted therethrough.
The branching waveguide is sometimes used with other waveguide(s) and fiber(s) connected thereto. In such a case, imprecision in manufacturing the products is prone to yield a light receiving waveguide of which the axis is out of alignment with the incident beam. Under the situation, a point of the peak of light intensity is varied, and resultantly, this brings about a varied branching ratio of the branching optical waveguide connected to the light receiving waveguide. Such a variation in the peak point of the light intensity is supposed to be caused by interference with high-order modes other than the basic mode that are excited within the optical waveguide because of the maladjustment of the axis. In general, the prior art branching optical waveguide is configured to take advantage of a property that the high-order modes are less effective in confining beams within the optical waveguide.
A first embodiment of the prior art branching waveguide has a configuration functioning to attenuate the variation in the peak point of light intensity by using a long straight light receiving waveguide.
A second embodiment of the prior art branching waveguide has a configuration including a light receiving waveguide along with a curved waveguide (see FIG. 12 and the descriptions of the same in Patent Document 1).
A third embodiment of the prior art branching waveguide is configured having a light receiving waveguide partially squeezed in shape. See FIG. 14 and the descriptions referred to the same in Japanese Patent No. 2809517.
The first, second and third prior art embodiments of the branching waveguide tend to attenuate the variation in the peak point of light intensity with the aid of radiations in the high-order mode, and this leads to an adverse effect of increased radiation loss. The first embodiment of the branching waveguide must be inadvertently long because of its configuration with the long straight light receiving waveguide for radiating beams in the high-order mode.
The second and third embodiments of the branching waveguide, although partially modified in waveguide shape and advantageous in radiating beams in high-mode so as to weaken the beam confining effect, still adversely radiate beams in the basic mode due to the modified shape of the waveguide. In order not to let the shape modified optical waveguide radiate beams in the basic mode, the contour of the shaped portion must be consistently gradual in terms of the direction of the beam propagation, and this results in an undesirably long structure of the waveguide. In either case, there is a common problem that the radiation loss becomes greater, and/or the longitudinal dimension is increased. As was mentioned above, the first embodiment of the prior art branching waveguide requires the long straight light receiving waveguide, and this is the cause to make the resultant waveguide product longer.
The present invention is made to overcome the aforementioned disadvantages of the prior art branching waveguides, and accordingly, it is an object of the present invention to provide a waveguide structure capable of facilitating to fabricate its parts and guaranteeing simple assembly/adjustment of the parts for stable final products.
It is another object of the present invention to provide a waveguide structure that enables full compensation for a deviation of the peak in the field distribution, that is superior in reproducibility but reduced in beam loss, and that can unify the branching ratio with greater accuracy.
It is still another object of the present invention to provide a waveguide structure that facilitates production of the waveguide without providing the waveguide with a contoured portion such as is squeezed in shape, and that radiates reduced beams in the high-order mode to decrease the radiation loss.
Throughout the specification, the term “connection/connecting” means optical coupling as in FIG. 2 where there may be a groove and gaps in the junction plane. The terms “axial zone” at a second beam incoming/outgoing end mean an area that covers a zone lower than a half of the amplitude of the oscillation, preferably, one third of the amplitude, and more preferably, one quarter of the amplitude in the case where the center of second and third waveguides is the center of oscillation in view of the peak point of beam intensity.
Also, the terms “the difference of refractive index” means a value given by the formula
            n      1      2        -          n      2      2            2    ⁢          n      1      2      where n1 is a refractive index of the core and n2 is that of the cladding layer.